The present invention relates to a method for the production of granular and especially spherical active carbon and to the use of the active carbon granules or spherules for various purposes.
Active carbon, due to its quite unspecific adsorptive properties, is the most-used adsorber. Legal requirements, but also increasing awareness of responsibility for the environment, are leading to a growing demand for active carbon.
Active carbon is generally obtained by low-temperature carbonization (pyrolysis) followed by activation of compounds containing carbon, those compounds being preferred which result in yields at reasonable cost, since the weight losses due to the elimination of volatile components during carbonization and to bum-off during activation are considerable.
But also the nature of the active carbons produced, whether finely or roughly porous, strong or brittle, depends on the starting material; common starting materials include coconut shells, wood wastes, peat, coal, tars, but also and especially plastics, which among other things play a certain part in the production of active carbon fabrics.
Active carbon is used in various forms: carbon powder, granular carbon, molded carbon, and, since the end of the seventies, spherical carbon. Spherical active carbon has a number of advantages over powder, split carbon and granular carbon, which makes it valuable or even indispensable for certain applications. Due to its special shape, but also due to its extremely high resistance to abrasion, spherical carbon is much in demand for special fields of application, such as surface filters for protective clothing against chemical poisons and filters for low concentrations of toxins in large amounts of air, for example. In charging reticulated, large-pore polyurethane foams with active carbon according to German DE-A1-38 13 563, only a free-flowing carbon is used when the inner layers of the foam material are also to be optimally filled. In the production of protective clothing against chemical toxins in line with DE-C3-33 04 349, only a very highly abrasion resistant carbon can be used, and only spherical carbon satisfies this requirement.
Spherical carbon is today produced mostly by a multiple-step and therefore very expensive, method. For example, in line with U.S. Pat. No. 1,468,982, the spherical carbon is made exclusively from bitumen in a multi-stage process. This multi-stage process is very expensive and the accordingly high price of this spherical carbon forestalls many applications in which it would be preferred on account of its properties.
It is therefore not surprising that various attempts have been made to produce a usable spherical carbon by another method.
Known in the state of the art is the low-temperature carbonization and activation of new or used ion exchangers which contain sulfone groups and the low-temperature carbonization of its intermediates in the presence of sulfuric acid, wherein the sulfone groups and/or sulfuric acid serve as crosslinkers. Such processes are described in DE-A-1-43 28 219 and in DE-A1-43 04 026, as well as in DE-A1-196 00 237, including the application for patent of addition DE 196 25 069.2. In these processes, however, particularly the large amounts of sulfur dioxide that are given off (about 1.5 kg SO.sub.2 per kg of end product), and the corrosion problems which it entails, are disadvantageous and problematical, so that possibilities for crosslinking other than by sulfuric acid appear desirable. In crosslinking with sulfuric acid, the yield is about 50% of the organic or polymeric material, regardless of whether the starting products are unsulfonated intermediates of ion exchangers or finished cation exchangers; and it is advantageous to promote crosslinking by slight air oxidation during the low-temperature carbonization and to facilitate subsequent activation, so that the formation of pseudographitic zones is suppressed.
EP-A-0 480 255 describes an agglomeration process for making a catalyst composed of carbon and nitrogen, wherein a loose carbon or active carbon powder is granulated together with polymerizable isocyanates. The isocyanates, in contrast to the present invention, are used only as "glue" or "adhesive," so to speak, in order to bond the initially loose carbon powder particles together, but not as starting substance for the carbon itself, as is the case in the present invention. Especially, the process according to EP-A-0 480 255 will not lead to spherules because the agglomeration is uncontrolled; any departure from the spherical form, however, necessarily results in lower resistance to abrasion. In contrast to EP-A-0 480 255, however, in the low-temperature carbonization and activation the teaching of the present application sets out from granular, especially spherical particles which largely retain their shape during the process. Also, in contrast to the present invention, according to EP-A-0 480 255 a product with a relatively large pore diameter of more than 100 nm is produced, due to the agglomeration, which consequently has no micropores at all, and due to its low BET surface area and its unfavorable ratio of pore volume to pore wall surface, also has poor adsorption properties and is entirely unsuitable for this purpose. Instead, the product of EP-A-0 480 255 is intended for use as catalyst. Also, due to its high graphite content of at least 90 wt.-%, which may indeed be necessary for catalysis, but not for adsorption, the products of EP-A-0 480 255 are entirely unsuitable for adsorption.